Colon x2 Marker for Markdown

This extends can introduce marker with colon to Markdown.

It is inspired by markdown-it/markdown-it-mark and bear.app.

Notation

::Notation::

If you customize token color

markup.bold.marker is specified as scope.

Ex.) settings

    "editor.tokenColorCustomizations": {
        "textMateRules": [
            {
                "scope": "markup.bold.marker",
                "settings": {
                    "fontStyle": "bold underline"
                }

            }
        ]
    }